Toolbar extension

The toolbar button can be reserved to open a Toolbar extension,
which allows more space for extra functions.

Toolbar extension buttons are for actions similar to those performed
by toolbar buttons, and the same types and rules apply. Extensions
should be used for accessing a set of logically similar options, for
example, when selecting content to be inserted into a message or for
adjusting imaging session-related values in the camera.

An extension can have 2 to 12 buttons (3 x 4 in portrait; 4 x 3
in landscape). The filling order follows the grid-filling rules, thus,
for example, the most important one should be placed first.